Container caps in the form of crown caps or caps for wide mouth vessels have been formed in the past by coating a surface-protecting paint on a metal sheet, forming the coated metal sheet into a crown shell or cap shell and then bonding a liner or packing material to the interface of the formed shell.
In premium sales of bottled drinks and the like, there is often adopted a system in which a purchaser returns a predetermined number of liners or a liner having a win mark printed thereon in return for which a prize is sent to the purchaser. In crown shells or caps for use in such premium sales, it is important that a liner or packing should be easily peelable from the crown shell or cap. Furthermore, from the sanitary viewpoint, it is important that any printed surface of a liner be out of direct contact with the packed drink. Since a liner per se is usually applied to the interface of a crown shell or cap shell in a melted state, it is preferred that when a liner is peeled from a cap, that any ink layer applied to the crown shell or cap shall be transferred to the peeled liner.
In order to provide caps meeting the above requirements, usually a plurality of coating layers are formed on a metal sheet constituting a cap shell and peeling is effected in the interface between two coated layers. However, coating layers forming the easily peelable interface are readily broken into small pieces during the steps of press or draw forming of a metal sheet into a crown shell or cap shell, during transport of the crown or cap shells, during capping and so-called roll-on processing, all of which results in objectionable dust particles being formed. This phenomenon is not preferred from a sanitary viewpoint and it reduces the commercial value of products.
It is therefore an object of our invention to provide for a cap having an easily peelable liner and which will not form objectionable dust particles during processing steps.